Oxidation of propane to give acrylic acid using catalysts in a mixture of crystalline phases

ABSTRACT

The invention relates to a method for the production of acrylic acid from propane, in which a gas mixture comprising propane, water vapour and, optionally, an inert gas and/or molecular oxygen is passed over a catalyst, comprising a crystalline catalyst phase of formula (I) or (I′) TeaMolVbNbcOx (I) Sba Mol VbOy (I′), associated with a crystalline catalyst phase for activating the propane.

This application is a national phase of International Application Ser.No. PCT/FR04/001290, filed May 25, 2004, published Dec. 9, 2004, whichclaims priority to French Application 03/06414 filed May 27, 2003, eachof which are incorporated by reference in their entireties herein, andfrom which priority is claimed.

The present invention relates to the selective oxidation of propane toacrylic acid, using catalysts in a mixture of crystalline phases, andthe preparation of these catalysts.

The yield and selectivity of the preparation of acrylic acid frompropane have often been rather limited, so that improvements are neededto increase the conversion of propane. The preparation of more activeand more selective catalysts serves to remedy this problem.

Patent application JP 10-330343 describes catalysts useful for preparingnitrites by oxidation of an alkane in the gas phase. These catalystswith a crystalline structure are represented by the formulaMo_(a)V_(b)Sb_(c)X_(x)O_(n) and defined by their lattice parameters anddiffraction angles (2θ). The symbol X denotes one or more metallicelements selected from Ti, Zr, Nb, Ta, Cr, W, Sn, etc. These catalystsare prepared by the addition of solutions or suspensions respectivelycontaining an antimony source and a vanadium source, followed by theaddition of a solution of suspension containing a specific quantity ofmolybdenum and addition of the element X in the powder or solutionstate. The oxides of these elements or derivatives, such as ammoniummetavanadate or ammonium paramolybdate, are particularly preferred. Themethod leads to a precursor which is dried and calcined to give acompound of metal oxides. Two phases may be obtained during thepreparation: an orthorhombic phase and a hexagonal phase. Theorthorhombic phase is the anticipated phase. Catalyst performance can beimproved by successive washings of the catalyst mixture obtained inorder to obtain the orthorhombic phase alone.

Patent application JP 7-232071 describes catalysts having a crystallinestructure corresponding to a formula of the MoVTeX type. These catalystsare precalcined at 300° C. The X-ray diffraction lines indicated tend toimply the presence of an orthorhombic lattice structure.

European patent application EP-A-608838 describes the preparation of anunsaturated carboxylic acid from an alkane by a vapor phase catalyticoxidation in the presence of a catalyst containing a mixed metal oxidecomprising Mo, V, Te and O as essential components, and at least oneelement selected from the group of niobium, tantalum, tungsten,titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium,cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron,indium and cerium, these elements being present in clearly definedproportions.

European patent application EP-A-895809 and U.S. Pat. No. 6,143,916describe oxide based catalysts comprising molybdenum, vanadium, niobium,oxygen, tellurium and/or antimony. These catalysts are used to convertpropane to acrylic acid in the presence of molecular oxygen (examples 9and 10 of the European application). Example 9 describes the oxidationof propane using a catalyst with the formulaMo₁V_(0.33)Nb_(0.1)Te_(0.22)O_(n) from a gas stream comprising propane,oxygen and helium and a steam stream. U.S. Pat. No. 6,143,916 describescrystalline forms of these catalysts.

It has now been found, this being the object of the present invention,that the mixture of several different phases of catalysts in thecrystalline state could yield surprising results for the oxidation ofpropane to acrylic acid, in comparison with the results obtained withcatalysts comprising a single phase. It has in fact been demonstratedthat the choice of the phases entering into the composition of thecatalyst used is a very important factor.

According to the invention, a crystalline catalyst phase based ontellurium or antimony and molybdenum, preferably with a hexagonallattice (referred to below as phase A) conferring the selectivity to thefinal mixture, in combination with a crystalline catalyst phase foractivating the propane, can yield highly unexpected oxidation resultsfrom the standpoint of activity and selectivity. A synergistic effectcan be observed in the use of the mixture of these crystalline catalystphases.

The phase based on tellurium or antimony and molybdenum, conferringselectivity to the final mixture, can be selected advantageously fromthe compounds of tellurium and/or molybdenum or the compounds ofantimony and molybdenum with a hexagonal lattice crystalline structure(phase A) or from Te₂MoO₇, or Te_(0.2)MoO_(x).

The crystalline catalyst phase for conferring good selectivity eithersatisfies the formula:Te_(a)Mo₁V_(b)Nb_(c)O_(x)  (I) orSb_(a′)Mo₁V_(b)O_(y)  (I′)in which

a is between 0.1 and 2 limits included;

a′ is between 0.1 and 2 limits included;

b is between 0 and 1 limits included;

c is between 0 and 0.2 limits included;

x and y represent the quantity of oxygen bound to the other elements anddepends on their oxidation states,

and corresponds to a hexagonal lattice structure of which the X-raydiffraction spectrum, diffraction angles (2θ) measured using the copperKα₁ and Kα₂ lines of as an X-ray source, with a 0.02° step, has a peakat the diffraction angle 28.2° and lattice parameters a=0.729 (±0.02)nm×p, p being an integer from 1 to 4; c=0.400 (±0.01) nm×q, q being aninteger from 1 to 2; α=90°, γ=120°,

or has a monoclinic structure Te₂MoO₇ or Te_(0.2)MoO_(x).

The crystalline catalyst phase for activating the propane is a phase ofcrystallized mixed metal oxides, more particularly based on molybdenumand vanadium, such as molybdenum-vanadium mixed oxides, such as ahexagonal phase catalyst (phase A) with antimony and niobium, or anorthorhombic phase catalyst (referred to below as phase B).

In particular, the crystalline phase for activating the propanesatisfies the formulae (II), (II′) or (II″):MO_(d)V_(e)O_(v)  (II)MO_(d′)V_(f)Sb_(g)Nb_(h)O_(w)  (II′)Mo_(d″)V_(i)Te_(j)Nb_(k)O_(z)  (II″)in which

-   -   d, d′ and d″ are between 0.93 and 1 limits included;    -   e is between 0.05 and 1 limits included;    -   f is between 0 and 0.5 limits included;    -   g is between 0.05 and 0.3 limits included;    -   h is between 0.01 and 0.2 limits included;    -   is between 0 and 0.5 limits included;    -   j is between 0.05 and 0.3 limits included;    -   k is between 0.01 and 0.2 limits included;    -   v, w and z represent the quantity of oxygen bound to the other        elements and depends on their oxidation states,

it being understood that the product of formula (II′) has either ahexagonal lattice structure in which the X-ray diffraction spectrum hasa peak at the diffraction angle 28.2°, and lattice parameters a=0.729(±0.02) nm×p, p being an integer from 1 to 4; c=0.400 (±0.01) nm×q, qbeing an integer from 1 to 2; α=90°, γ=120°, or has an orthorhombiclattice structure in which the X-ray diffraction spectrum has a peak atthe diffraction angle 27.3° and lattice parameters a=2.68 (±0.04) nm;b=2.12 (±0.04) nm; c=0.401 (±0.006) nm×q′, q′ being an integer from 1 to2; α=β=γ=90°, and it being understood that the product of formula (II″)has an orthorhombic lattice structure and also has an X-ray diffractionpeak at the diffraction angle 27.3° and lattice parameters a=2.68(±0.04) nm; b=2.12 (±0.04) nm; c=0.401 (±0.006) nm×q′, q′ being aninteger from 1 to 2; α=γ=90°.

In the X-ray diffraction spectra of the above hexagonal or orthorhombiccrystalline structures, the diffraction angles (2θ) are measured usingthe copper Kα₁ and Kα₂ lines as an X-ray source, with a step of 0.02°.

According to a preferred embodiment of the invention, the combination ofcrystalline catalyst phases is prepared in ratios from 90/10 to 15/85 byweight of the total weight of mixture, of the catalyst conferring goodselectivity/catalyst for activating the propane, preferably 90/10 to50/50 by weight of the total weight of mixture, and particularlypreferred, from 70/30 to 50/50 by weight of the total weight of mixture,of the catalyst conferring good selectivity/catalyst for activating thepropane.

Thus in particular, in the combination of catalysts, it is advantageousto use 10 to 50% by weight of the crystalline catalyst phase foractivating the propane, preferably 30 to 50% by weight.

A subject of the present invention is a method for preparing acrylicacid from propane, in which a gas mixture comprising propane, steam and,optionally, an inert gas and/or molecular oxygen, is passed over acatalyst for conferring good selectivity for acrylic acid, of formula(I), (I′), Te₂MoO₇ or Te_(0.2)MoO_(x) combined with a crystallinecatalyst phase for activating the propane.

Preferably, the method according to the present invention consists inpassing the above gas mixture over a catalyst comprising a combinationof a catalyst with formula (I), (I′), Te₂MoO₇ or Te_(0.2)MoO_(x) and acrystalline catalyst phase of formula (II), (II′) or (II″).

According to a preferred embodiment of the invention, acrylic acid isprepared from propane using a catalyst comprising a combination ofcrystalline phases in ratios from 90/10 to 15/85 by weight of the totalweight of mixture, of the catalyst conferring good selectivity/catalystfor activating the propane, preferably 90/10 to 50/50 by weight, andparticularly preferred, from 70/30 to 50/50 by weight of the totalweight of mixture, of the catalyst conferring good selectivity/catalystfor activating the propane.

Preferably, according to the method of the present invention, whenoperating in the presence of molecular oxygen, the propane/molecularoxygen molar ratio in the initial gas mixture is equal to or greaterthan 0.5. A molecular ratio equal to or greater than 0.3 may also beadvantageous.

The present invention further relates to the use of a combination ofcatalysts with a crystalline structure of formula (I), or (I′), Te₂MoO₇or Te_(0.2)MoO_(x) with catalysts of a crystalline structure of formulae(II), (II′) or (II″) for activating the propane, for preparing acrylicacid from propane.

The method according to the invention simultaneously provides goodselectivity for acrylic acid and a high propane conversion. Furthermore,it can be implemented easily in a fixed bed, fluidized bed or movingbed, and the reactants can be injected into the reactor at variouspoints, so that the operation remains outside the flammability zonewhile producing a high propane concentration, and in consequence, highcatalyst productivity. The unconverted propane can be recycled.

According to a particularly advantageous embodiment, the methodaccording to the invention comprises the following steps:

I/ In the Absence of Molecular Oxygen

If the initial gas mixture contains no molecular oxygen, the propane isoxidized by the following redox reaction (A):SOLID_(oxidized)+PROPANE→SOLID_(reduced)+ACRYLIC ACID  (A)II/ In the Presence of Molecular Oxygen

-   -   a) the initial gas mixture is introduced into a reactor with a        moving catalyst bed;    -   b) the gases are separated from the catalyst at the outlet of        the first reactor;    -   c) the combination of catalysts is sent to a regenerator; and    -   d) the regenerated catalyst issuing from the regenerator is        reintroduced into the reactor.

According to another advantageous embodiment of the invention, themethod comprises the repetition, in a reactor provided with thecombination of catalysts, of the cycle comprising the followingsuccessive steps:

-   -   1) injection of the gas mixture as previously defined;    -   2) injection of steam and, if applicable, inert gas;    -   3) injection of a mixture of molecular oxygen, steam and, if        applicable, inert gas; and    -   4) injection of steam and, if applicable, inert gas.

It is understood that step 1) can be carried out in the form of multipleinjections.

According to an improvement of the advantageous embodiment justdescribed, the cycle comprises an additional step that precedes orfollows step 1) and during which the gas mixture corresponding to thatof step 1) is injected, but without molecular oxygen, thepropane/molecular oxygen molar ratio then being calculated on the wholefor step 1) and this additional step.

According to an advantageous embodiment of the improvement justdescribed, the additional step precedes step 1) in the cycle.

Other features and advantages of the invention are described in greaterdetail below.

DETAILED DESCRIPTION OF THE METHOD OF THE INVENTION

According to the invention, in the alternatives in which molecularoxygen is introduced, since the propane/molecular oxygen molar ratio inthe initial gas mixture is preferably equal to or greater than 0.5 orequal to or greater than 0.3, the conversion of propane to acrylic acidusing the catalyst is carried out by oxidation, probably by thefollowing competing reactions (A) and (B):

-   -   the conventional catalytic reaction (B):        CH₃—CH₂—CH₃+2O₂→CH₂═CH—COOH+2H₂O  (B)    -   and the redox reaction (A) mentioned above:        SOLID_(oxidized)+CH₃—CH₂—CH₃→SOLID_(reduced)+CH₂═CH—COOH  (A)

The propane/steam volume ratio in the initial gas mixture is notcritical and may vary within wide limits.

Similarly, the proportion of inert gas, which may be helium, krypton, amixture thereof, or nitrogen, carbon dioxide, etc., is also not criticaland may also vary within wide limits.

The proportions of the components of the initial gas mixture aregenerally as follows (in molar ratios):

propane/oxygen/inert gas (He—Kr)/H₂O (steam)=1/0.05-2/1-10/1-10 orpreferably 1/0.5-2/1-10/1-10

Even more preferably, they are 1/0.1-1/1-5/1-5.

Even more preferably they are 1/0.167-0.667/2-5/2-5. The followingproportions are also particularly advantageous: 1/0.2-0.4/4-5/4-5.

In general, reactions (A) and (B) are conducted at a temperature ofbetween 200 and 500° C., preferably between 250 and 450° C., even morepreferably between 350 and 400° C. The pressure in the reactor orreactors is generally 1.01×10⁴ to 1.01×10⁶ Pa (0.1 to 10 bar),preferably 5.05×10⁴ to 5.05×10⁵ Pa (0.5-5 bar).

The residence time in the reactor is generally between 0.01 and 90seconds, and preferably between 0.1 and 30 seconds.

Preparation of Catalysts

The crystallized catalysts of formulae (I), (I′), Te₂MoO₇ orTe_(0.2)MoO_(x) or formulae (II), (II′), (II″) can be prepared byvarious methods, such as hydrothermal synthesis, coprecipitation orsolid-solid reaction.

Some of them can be prepared in particular by the methods described inU.S. Pat. No. 6,143,916, in Japanese patent application JP 10-330343, inJ. of Solid State Chemistry, 129, 303 (1997); Acta Chemica Scandinavica,26, 1827 (1972); Applied Catalysis, A: General, 244, 359-70 (2003); ordescribed by L. M. Plyasova et al, Kinetica i Kataliz, 31(6), 1430-1434(1990); by A. Kaddouri et al, J. Therm. Anal. Cal, 66, 63-78 (2001); byJ. M. M. Millet et al, Appl, Catal., 232, 77-92 (2202). They can also beprepared as described below in the examples.

In general, the sources of the various metals used as raw materials areoften oxides, but are not necessarily limited to oxides. In anonlimiting manner, the following raw materials can be used:

-   -   in the case of molybdenum, ammonium molybdate, ammonium        paramolybdate, ammonium heptamolybdate, molybdic acid,        molybdenum halides and oxyhalides such as MoCl₅, organometallic        compounds of molybdenum such as molybdenum alkoxides such as        Mo(OC₂H₅)₅, acetylacetone molybdenyl;    -   in the case of tellurium, the tellurium, telluric acid, TeO₂;    -   in the case of antimony, for example, antimony oxide (antimony        trioxide), particularly the senarmontite variety, antimony        sulfate (Sb₂(SO₄)₃) or an antimony chloride (an antimony        trichloride, antimony pentachloride);    -   in the case of vanadium, ammonium metavanadate, vanadium halides        and oxyhalides such as VCl₄, VCl₅ and VOCl₃, organometallic        compounds of vanadium such as vanadium alkoxides such as        VO(OC₂H₅)₃;    -   in the case of niobium, niobic acid, niobium tartrate, niobium        hydrogenoxalate, ammonium oxotrioxalate niobiate        {(NH₄)₃[NbO—(C₂O₄)₃], 1.5H₂O}, niobium ammonium oxalate, niobium        oxalate or tartrate, niobium halides or oxyhalides such as        NbCl₃, NbCl₅ and organometallic compounds of niobium such as        niobium alkoxides such as Nb(OC₂H₅)₅, Nb(O-n-Bu)₅;        and, in general, all compounds suitable for forming an oxide by        calcination, that is, metal salts of organic acids, metal salts        of inorganic acids, complex metallic compounds, etc.

One mode for preparing catalysts consists in mixing, with stirring,aqueous solutions of niobic acid, oxalic acid, and ammoniumheptamolybdate, ammonium metavanadate, telluric acid or antimony oxide,and preferably precalcining the mixture in air at about 300-320° C., andcalcining under nitrogen at about 600° C.

According to a preferred embodiment, one method for preparing catalystsconsists in preparing a solution of niobic acid and oxalic acid,preparing a solution of molybdenum, vanadium, tellurium or antimony,mixing the two solutions to form a gel, followed by drying of the gelobtained, precalcination and calcination.

According to a particularly preferred method, the catalyst can beprepared by the following steps:

1) dissolution in water of a vanadium source, for example, ammoniummetavanadate, with stirring and, optionally, heating;

2) if applicable, addition to the above solution of a source oftellurium or antimony, for example, telluric acid, or antimony oxide(particularly the senarmontite variety);

3) addition of a molybdenum source, for example ammonium heptamolybdate;

4) reaction of the solution obtained, under reflux;

5) if applicable, addition of an oxidant such as hydrogen peroxide inthe case of antimony catalysts;

6) if applicable, addition of the solution prepared by mixing, withheating, a niobium source, for example niobic acid, with oxalic acid;

7) reaction of the reaction medium under reflux and preferably underinert atmosphere, until a gel is obtained;

8) drying of the gel obtained;

9) preferably, precalcination of this gel; and

10) calcination of the gel, precalcined if necessary, to obtain thecatalyst.

In the alternatives to the above methods:

drying [for example in step 8)] can be carried out in an oven as a thinlayer, by spray drying, by freeze drying, by zeodration, by microwaves,etc;

precalcination can be carried out under air flow at 280-300° C. or understatic air at 320° C., in fluidized bed, in a rotary furnace in anaerated fixed bed, so that the catalyst particles are separated from oneanother to prevent them from joining during precalcination or possiblyduring calcination;

calcination is preferably carried out under very pure nitrogen and at atemperature close to 600° C., for example in a rotary furnace or in afluidized bed and for a period of up to 2 hours.

According to a particularly preferred embodiment of the invention,precalcination is carried out:

-   -   either at a temperature below 300° C. under an air flow of at        least 10 ml/min/g of catalyst;    -   or at a temperature of between 300 and 350° C. under an air flow        of less than 10 ml/min/g of catalyst.

According to a particularly preferred embodiment, precalcination iscarried out:

-   -   at about 320° C. under an air flow of less than 10 ml/min/g; or    -   at about 290° C., under an air flow of about 50 ml/min/g.

According to another method for preparing catalysts, a solid-solidreaction is carried out by mixing the metal sources followed byco-grinding to obtain a uniform mixture. The solid is obtained afterheating under reduced pressure at a temperature close to 600° C.

Advantageously, metal oxides or the metal itself are used as the metalsource. Preferably, the heating is carried out for a prolonged period(preferably 3 days to 1 week).

The catalysts prepared by the methods described above may each beproduced in the form of particles generally from 20 to 300 microns indiameter, the particles of each of the combined catalysts generallybeing mixed before carrying out the method according to the invention.The particles can be obtained by spray-drying a gel or a suspension.

The combination of catalysts may also be in the form of a solidcatalytic composition comprising particles each of which comprises bothof the catalysts.

Catalyst Regeneration

During the redox reaction (A), the catalysts undergo a reduction and aprogressive loss of their activity. When the catalysts are at leastpartially in the reduced state, they are regenerated by the reaction(C):SOLID_(reduced)+O₂→SOLID_(oxidized)  (C)by heating in the presence of oxygen or a gas containing oxygen at atemperature of 250 to 500° C., for the time necessary to reoxidize thecatalysts.

The proportions of the components of the regeneration gas mixture aregenerally as follows (in molar ratios):

oxygen/inert gas (He—Kr)/H₂O (steam)=1/1-10/0-10 Preferably, they are1/1-5/0-5.

Instead of using oxygen alone, dry air (21% O₂) can be used. Instead ofor in addition to steam, moist air can be used.

The regeneration temperature is generally 250 to 500° C.

The method is generally carried out until the catalyst reduction rate isbetween 0.1 and 10 g of oxygen per kg of catalyst.

This reduction rate can be monitored during the reaction by the quantityof products obtained. The equivalent quantity of oxygen is calculated.It can also be monitored by the exothermicity of the reaction. Thereduction rate can also be tracked by the quantity of oxygen consumed inthe regenerator.

After regeneration, which can be carried out under temperature andpressure conditions identical to or different from those of reactions(A) and (B), the catalysts recover an initial activity and can bereintroduced into the reactor.

Reactions (A) and (B) and regeneration (C) can be carried out in aconventional reactor, such as a fixed bed reactor, a fluidized bedreactor, or a moving bed reactor.

Thus, reactions (A) and (B) and regeneration (C) can be carried out in atwo-stage device, that is, a reactor and a regenerator which operatesimultaneously, and in which two feeds of the catalyst combinationalternate periodically.

Reactions (A) and (B) and regeneration (C) can also be carried out inthe same reactor, by alternating the reaction and regeneration periods.

Preferably, reactions (A) and (B) and regeneration (C) are carried outin a moving catalyst bed reactor, particularly in a vertical reactor,with the catalyst preferably moving upward.

An operating mode with a single pass of the gases or with gas recyclingcan be used.

According to a preferred embodiment, the propylene produced and/or theunreacted propane are recycled (or returned) to the reactor inlet, thatis, they are reintroduced at the reactor inlet, in a mixture or inparallel with the initial mixture of propane, steam and, optionally,inert gas(es).

The present invention has the great advantage of combining very goodselectivity for acrylic acid and good propane conversion, because of thecombination of catalysts employed and the synergistic effect procured.In this synergistic effect, it may be observed, on the one hand, thateach catalyst considered separately is less efficient than thecombination of the catalyst for procuring good selectivity with thecatalyst for activating the propane and, on the other hand, that theselectivity observed is higher than the additive effect procured by thetwo catalysts considered separately, in nearly all cases. This effectcan be observed in particular in the tests discussed below.

EXAMPLES

The following examples illustrate the present invention but withoutlimiting its scope.

In the examples below, the selectivities and the propane conversion aredefined as follows:

$\begin{matrix}{Propane} \\{{Conversion}\mspace{11mu}(\%)}\end{matrix} = {\frac{{Number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{reacted}}{{Number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{introduced}} \times 100}$$\begin{matrix}{{Acrylic}\mspace{14mu}{acid}} \\{{selectivity}\mspace{11mu}(\%)}\end{matrix} = {\ldots\mspace{14mu}\frac{{Number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{acrylic}\mspace{14mu}{acid}\mspace{14mu}{formed}}{{Number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{reacted}} \times 100}$

The selectivities relative to the other compounds are calculatedsimilarly.

Preparation of Pure Catalyst Phases

Example 1

Preparation of Phase A with Tellurium of CompositionMoV_(0.8)Te_(0.6)O_(x)

The preparation was made by solid-solid reaction in an under vacuumsealed ampoule. 10.00 g of MoO₃ (Merck), 1.37 g of molybdenum metal(Alfa Aesar), 8.01 g TeO₂ (Alfa Aesar) and 3.04 g V₂O₅ (Riedel de Haën)were ground together in an agate mortar for 15 minutes, until a uniformmixture was obtained. This mixture was introduced into a quartz ampoule.The ampoule was then sealed under vacuum and heated at 600° C. for oneweek. The solid recovered was analyzed by X-ray diffraction. Theanalysis confirmed the production of the desired phase, whichcorresponded to the hexagonal structure (diffraction diagram—FIG. 1).The solid obtained had the chemical formula: MoV_(0.8)Te_(0.6)O_(x),where x is the quantity of oxygen corresponding to the oxidation stateof the cations.

Example 2

Preparation of Phase A with Tellurium and Niobium of CompositionMoV_(0.3)Te_(0.4)Nb_(0.1)O_(x)

Phase A with tellurium containing niobium was obtained bycoprecipitation. 5.00 g of ammonium heptamolybdate (Starck)+1.00 g ofammonium metavanadate (GFE)+2.60 g of telluric acid (Fluka)+25 ml ofwater were introduced into a beaker. The beaker was heated (70° C.) withstirring until a clear solution was obtained. Simultaneously, 0.52 g ofniobic acid (CBMM)+1.08 g of oxalic acid (Alfa Aesar)+15 ml of waterwere introduced into a beaker. The beaker was heated until the solutionbecame clear (about 4 hours, temperature 70° C.), the solution wascentrifuged (3500 rpm for 15 minutes) and the liquid phase was added tothe solution containing Mo, V and Te. This produced an orange gel thatwas placed overnight in the oven at 110° C. The solid obtained wasprecalcined in air for 4 hours at 300° C. (50 ml/min/g) and calcined for2 hours at 600° C. under nitrogen (50 ml/min/g). The solid obtained hadthe chemical formula: MoV_(0.3)Te_(0.4)Nb_(0.1)O_(x). The solidrecovered was analyzed by X-ray diffraction (FIG. 2).

Example 3

Preparation of Phase A with Antimony of CompositionMo₁V_(0.5)Sb_(0.3)O_(y)

Phase A with antimony was prepared like the one in example 1, but withthe following components,

15.00 g of MoO₃ (Merck)+5.49 g Sb₂O₃+2.85 g of V₂O₅ (Riedel deHaën)+2.05 g Mo (Alfa Aesar) were ground for 15 minutes in an agatemortar and introduced into an ampoule. The ampoule was sealed undervacuum and heated at 600° C. for one week. The solid obtained had thechemical formula: Mo₁V_(0.5)Sb_(0.3)O_(y). The solid recovered wasanalyzed by X-ray diffraction (FIG. 3).

Example 4

Preparation of Phase A with Antimony and Niobium of CompositionMoV_(0.3)Sb_(0.1)Nb_(0.1)O_(w).

Phase A with antimony containing niobium was obtained bycoprecipitation. 7.00 g of ammonium heptamolybdate (Starck)+1.39 g ofammonium metavanadate (GfE) were introduced into a beaker, heated (80°C.) with stirring until a clear solution was obtained. 1.17 g of Sb₂O₃(Alfa Aesar) was then added and the mixture stirred for four hours withcontinued heating. 2 ml of H₂O₂ containing 30% by weight (Alfa Aesar)diluted in 10 ml of water was then added, and the solution then turned aclear orange color.

Simultaneously, 0.66 g of niobic acid (CBMM)+1.34 g of oxalic acid (AlfaAesar)+15 ml of water were introduced into a beaker. The mixture washeated until the solution became clear (about 4 hours, temperature 70°C.), and then centrifuged (3500 rpm for 15 minutes) and the liquid phasewas then added to the solution containing Mo, V and Sb. This produced ayellow gel that was left overnight in the oven at 110° C. The solidobtained was precalcined in air for 4 hours at 300° C. (50 ml/min/g) andcalcined for 2 hours at 600° C. under nitrogen (50 ml/min/g). The solidobtained had the chemical formula: MoV_(0.3)Sb_(0.1)Nb_(0.1)O_(w).

The solid recovered was analyzed by X-ray diffraction (FIG. 4).

Example 5

Preparation of a Phase with Molybdenum of the V_(0.95)Mo_(0.97)O₅ Type

The V_(0.95)Mo_(0.97)O₅ phase was prepared by hydrothermal synthesis.2.00 g of ammonium heptamolybdate (Starck), 1.33 g VOSO₄ (Alfa Aesar)and 0.07 g of NH₄OH (28% by weight NH₃) were introduced with 50 ml ofwater into a 100 ml Teflon jar. The mixture was left for 72 hours at175° C. in an autoclave. The solid was then filtered, washed withdistilled water, dried in the oven at 110° C. and calcined undernitrogen at 600° C. for 2 hours (50 ml/min/g). The solid obtained had achemical formula of the Mo₁V₁O_(v) type. The solid recovered wasanalyzed by X-ray diffraction (FIG. 5), and conformed to JCPDS (JointCommittee of Powder Diffraction Spectroscopy) datasheet 77-0649. Thisphase has been described by L. M. Plyasova et al, Kinetica i kataliz,31(6), 1430-1434 (1990).

Example 6

Preparation of a Phase with Tellurium and Molybdenum of the Te₂MoO₇ Type

The Te₂MoO₇ phase was prepared by coprecipitation. 6.50 g of telluricacid (Fluka) and 2.50 g ammonium heptamolybdate (Starck) were dissolvedin a minimum of water (15 ml). The mixture was heated (80° C.) withstirring and allowed to evaporate to a white paste that was left to dryovernight in the oven at 110° C. The solid obtained was calcined for 2hours at 470° C. in air (50 ml/mm/g). The solid obtained had thechemical formula of Mo_(0.5)Te₁O_(x). The solid recovered was analyzedby X-ray diffraction (FIG. 6) and conformed to JCPDS datasheet 70-0047.This phase has been described by A. Kaddouri et al, J. Therm. Anal.Cal., 66, 63-78 (2001).

Example 7

Preparation of a Phase with Tellurium and Niobium of Composition:Mo₁V_(0.26)Te_(0.10)Nb_(0.14)O₂

MoVTeNb catalyst containing a high concentration of phase B. Thefollowing were introduced simultaneously into a 100 ml beaker: 35 ml ofdistilled water+7.78 g of ammonium heptamolybdate (Starck)+1.70 g ofammonium metavanadate (GfE)+2.22 g of telluric acid (Fluka). The mixturewas heated at 80° C. with stirring to obtain a clear red solution. Thesolution was then left to cool at ambient temperature.

A solution of niobic acid/oxalic acid with an oxalate/Nb ratio of 2.70was prepared simultaneously. The following were introduced into a 50 mlbeaker: 10 ml of distilled water+0.82 g of niobic acid (CBMM)+1.67 g ofoxalic acid (Alfa Aesar). The mixture was heated at 70° C. with stirringuntil the initial solution became clear (about 4 hours). The solutionwas centrifuged (3500 rpm for 15 minutes) and the liquid phase was thenintroduced into the clear red solution containing the molybdenum,vanadium and tellurium. After a few minutes, an opaque orange gel wasobtained, and was placed in a crystallizer for drying overnight in theoven at 110° C. The solid was precalcined in air at 300° C. for 4 hours(50 ml/min/g) and then calcined in purified nitrogen at 600° C. for twohours (50/ml/min/g).

The solid recovered was analyzed by X-ray diffraction. This showed amixture of hexagonal phase and the desired orthorhombic phase. The solidobtained had the chemical formula Mo₁V_(0.26)Te_(0.10)Nb_(0.14)O_(z) andhad a diffraction diagram similar to the one described by J. M. M.Millet et al, Appl. Catal., 232 77-92 (2002).

The solid obtained was washed in a solution of hydrogen peroxide (AlfaAesar) containing 30% by weight diluted twice, for 4 hours, at ambienttemperature. The solution was filtered and the solid recovered dried inthe oven (110° C.) and then calcined for 2 hours in nitrogen at 600° C.(50 ml/min/g). The solid recovered was analyzed by X-ray diffraction(FIG. 7). The analysis confirmed that the desired phase was obtained,corresponding to the orthorhombic structure like the one described inthe above publication with a small quantity of hexagonal phase. Thesolid obtained had the chemical formulaMo₁V_(0.26)Te_(0.10)Nb_(0.14)O_(z).

Example 8

Preparation of a Phase with Antimony and Niobium of CompositionMoV_(0.28)Sb_(0.43)Nb_(0.15)O_(w).

MoVSbNbO catalyst containing a high concentration of phase B. Thefollowing were introduced into a flask: 1.99 g of ammonium metavanadate(GfE) and 45 ml of distilled water. The mixture was heated under refluxat 95° C. with stirring to obtain a clear solution, after which 1.24 gof antimony trioxide (Alfa Aesar)+10.00 g of ammonium heptamolybdate(Starck) were added. Heating was continued for one hour with argonblanket. A solution containing 2 ml of hydrogen peroxide (Alfa Aesar)containing 30% by weight for 10 ml of water was introduced. A clearorange solution was then obtained.

A mixture of niobic acid/oxalic acid with an oxalate/Nb ratio of 2.7 wasprepared simultaneously. The following were introduced into a 50 mlbeaker: 1.73 g of oxalic acid (Alfa Aesar), 0.76 g of niobic acid (CBMM)and 15 ml of distilled water. The mixture was heated at 70° C. withstirring until the initial solution became clear (about 4 hours). Thesolution was then centrifuged (3500 rpm for 15 minutes) and the liquidphase introduced into the solution containing the molybdenum, vanadium,and antimony. The mixture was stirred for half an hour and then placedin the oven at 110° C. for drying. The solid was precalcined in air at320° C. for 4 hours (temperature ramp 2.5° C./min), flow rate=0ml/min/g) and then calcined under purified nitrogen at 600° C. for 2hours (temperature ramp 2.5° C./min, flow rate=50 ml/mn/g).

The solid recovered was analyzed by X-ray diffraction. This revealed amixture of hexagonal phase and the desired orthorhombic phase. The solidobtained had the chemical formula MoV_(0.3)Sb_(0.15)Nb_(0.1)O_(w).

The solid obtained was washed in a solution of hydrogen peroxide (AlfaAesar) containing 30% by weight diluted twice, for 4 hours, at ambienttemperature. The solution was filtered and the solid recovered dried inthe oven, then calcined for 2 hours in nitrogen at 600° C. (50ml/min/g). The solid recovered was analyzed by X-ray diffraction (FIG.8). The analysis confirmed the production of the desired phase, whichcorresponds to the orthorhombic structure. The solid had the compositionMoV_(0.28)Sb_(0.13)Nb_(0.15)O_(w).

Example 9

Preparation of a Phase with Tellurium and Molybdenum of the TeMo₅O₁₆Type

The TeMo₅O₁₆ phase was obtained by solid-solid reaction in a sealedampoule under vacuum. 28.02 of g MoO₃ (Merck), 1.32 g of molybdenummetal (Alfa Aesar) and 6.65 g TeO₂ (Alfa Aesar) were ground together inan agate mortar for 15 minutes, until a uniform mixture was obtained.This mixture was introduced into a quartz ampoule. The ampoule was thensealed under vacuum and heated at 600° C. for 72 hours. The solidrecovered was analyzed by X-ray diffraction (FIG. 9), and conformed todatasheet JCPDS 70-0451. The analysis confirmed the production of thedesired phase, which corresponded to the monodinic structure having thechemical formula MoTe_(0.2)O_(x).

Example 10

Preparation of a Phase of Composition MoV_(0.23)Te_(0.09)Nb_(0.16) of aMoVTeNb Catalyst Containing a Large Amount of Phase B.

The following were introduced simultaneously into a 100 ml beaker: 35 mlof distilled water+7.78 g of ammonium heptamolybdate (Starck)+1.70 g ofammonium metavanadate (GfE)+2.22 g of telluric acid (Fluka). The mixturewas heated at 80° C. with stirring until a clear red solution wasobtained. This solution was allowed to cool at ambient temperature.

A solution of niobic acid/oxalic acid with an O_(x)/Nb ratio of 2.70 wasprepared simultaneously. The following were introduced into a 15 mlbeaker: 10 ml of distilled water+0.82 g of niobic acid (CBMM)+1.67 g ofoxalic acid (Alfa Aesar). The mixture was heated at 70° C. with stirringuntil the initial solution became clear (about 4 hours). This solutionwas centrifuged (3500 rpm for 15 minutes) and the liquid phaseintroduced into the clear red solution containing the molybdenum,vanadium and tellurium. After a few minutes, an opaque orange gel wasobtained and placed in a crystallizer for drying overnight in the ovenat 110° C. The solid was precalcined in air at 300° C. for 4 hours (50ml/min/g) and then calcined under purified nitrogen at 600° C. for 2hours (50 ml/min/g).

The solid recovered was analyzed by X-ray diffraction. This revealed amixture of hexagonal phase and the desired orthorhombic phase. The solidobtained had a diffraction diagram similar to the one described inpublication J. M. M. Millet, H. Roussel, A. Pigamo, J. L. Dubois, J. C.Jumas, Appl. Catal 232 (2002) 77-92, FIG. 1 b. The solid obtained hadthe chemical formula MoV_(0.3)Te_(0.2)Nb_(0.1).

The solid obtained was washed in a solution of hydrogen peroxide (AlfaAesar) containing 30% by weight diluted twice for 4 hours at ambienttemperature. The solution was filtered and the solid recovered dried inthe oven (at a 110° C.) and then calcined for 2 hours in nitrogen at600° C. (50 mL/min/g). The solid recovered was analyzed by X-raydiffraction (FIG. 10). The analysis confirmed that the production of thedesired phase, which corresponded to the orthorhombic structure asdescribed in the above publication with a small quantity of hexagonalphase. The solid obtained had the chemical formulaMo₁V_(0.23)Te_(0.09)Nb_(0.16)O_(z).

Example 11

Preparation of a Phase of Composition MoV_(0.24)Te_(0.9)Nb_(0.16) of aMoVTeNb Catalyst Containing a Large Amount of Phase B.

The procedure described in Example 7 above was followed.

The following were introduced simultaneously into a 100 ml beaker: 35 mlof distilled water+7.78 g of ammonium heptamolybdate (Starck)+1.70 g ofammonium metavanadate (GfE)+2.22 g of telluric acid (Fluka). The mixturewas heated at 80° C. with stirring to obtain a clear red solution. Thesolution was then left to cool at ambient temperature.

A solution of niobic acid/oxalic acid with an Ox/Nb ratio of 2.70 wasprepared simultaneously. The following were introduced into a 50 mlbeaker: 10 ml of distilled water+0.82 g niobic acid (CBMM)+1.67 g ofoxalic acid (Alfa Aesar). The mixture was heated at 70° C. with stirringuntil the initial solution became clear (about 4 hours). This solutionwas centrifuged (3500 rpm for 15 minutes) and the liquid phase was thenintroduced into the clear red solution containing the molybdenum,vanadium and tellurium. After a few minutes, an opaque orange gel wasobtained, and was placed in a crystallizer for drying overnight in theoven at 110° C. The solid was precalcined in air at 300° C. for 4 hours(50 ml/min/g) and then calcined in purified nitrogen at 600° C. for 2hours (50 ml/min/g).

The solid recovered was analyzed by X-ray diffraction. This revealed amixture of hexagonal phase and of the desired orthorhombic phase. Thesolid obtained had a diffraction diagram similar to the one described inpublication J. M. M. Millet, H. Roussel, A. Pigamo, J. L. Dubois, J. C.Jumas, Appl. Catal 232 (2002) 77-92, FIG. 1 b. The solid obtained hadthe chemical formula MoV_(0.3)Te_(0.2)Nb_(0.1).

The solid obtained was washed in a solution of hydrogen peroxide (AlfaAesar) containing 30% by weight diluted twice, for 4 hours, at ambienttemperature. The solution was filtered and the solid recovered dried inthe oven (110° C.) and then calcined for 2 hours in nitrogen at 600° C.(50 ml/min/g). The solid recovered was analyzed by X-ray diffraction.The analysis confirmed that the desired phase was obtained,corresponding to the orthorhombic structure like the one described inthe above publication with a small quantity of hexagonal phase. Thesolid obtained had the chemical formula MoV_(0.24)Te_(0.09)Nb_(0.16).

Catalyst Test Example 12

The pure phases thus prepared were tested as follows: 0.5 to 1.5 g ofsolid was introduced into a straight fixed-bed Pyrex reactor and thetemperature ramp (2.5° C./min) was carried out under nitrogen. When thedesired temperature was reached, the following reaction mixtureconditions were obtained: total flow rate 30 ml/min (5% C₃H₈, 5% Ne, 10%O₂, 45% H₂O and 35% N₂ (molar percent)) and the reactor was allowed tostabilize for 30 minutes. A 25 ml flask containing 5 ml of water wasplaced at the reactor outlet to allow the organic compounds to condense.For each temperature, the condensation time was 2 hours. Incondensableswere analyzed in line by a Chrompack chromatograph and the liquideffluents were analyzed after reaction on another Chrompackchromatograph.

A diagram of the reactor is appended (FIG. 11).

Example 13

To perform the tests of the catalyst 10 and 11: tests A1, A2, AA1, AA2under the same conditions of tests I and J of catalyst 7, a mass of 0.5g was used for the tests A1 and AA1, and a mass of 0.47 g was used forthe tests A2 and AA2. The results obtained are given in Table 2. Theresults indicate very good reproducibility of catalyst performance.

TABLE 1 Table of mechanical mixtures of phases (mass of each solid inthe mixture): Mass of solid in the column (solid C) + mass of solid onthe line (solid L). The letter indicated in the table corresponds to thenumeral of the test example. Mixture: Solid C + Solid L 1 2 3 4 5 6 7 89 10 11 1 D: 1 g 2 E: 1 g 3 F: 1 g 4 T: 0.5 + 0.5 g G: 1 g 5 P: 0.5 +0.5 g Q: 0.5 + 0.5 g B: 0.5 g Y: 0.5 + 0.1 g Z: 0.5 + 0.25 g 6 R: 0.5 +C: 1 g 0.5 g 7 M: 0.5 + 0.5 g O: 0.5 + 0.5 g S: 0.5 + 0.5 g N: 0.5 + I:0.5 g X: 0.4 + 0.1 g 0.5 g J: 0.47 g W: 0.5 + 1 g 8 L: 0.5 + 0.5 g H:0.5 g 9 U: 0.5 + V: 0.5 + K: 1 g 0.5 g 0.5 g 10 M: 0.5 + 0.5 g O: 0.5 +0.5 g N: 0.5 + A1: 0.5 0.5 g A2: 0.47 11 AA1: 0.5 AA2: 0.47

Example D: 1 g of solid of example 1 was fed to a reactor. The solid washeated under nitrogen to the desired temperature. The catalyst was thenplaced in the reaction mixture: total flow rate=30 ml/min. (5% Ne, 10%O₂, 35% N₂, 45% H₂O and 5% C₃H₈). Examples B, C and E to K: a mass m(specified in the table) of the solids prepared in examples 2 to 9 wasfed to the reactor as in example D, and the catalyst was tested as inexample D.

Examples L to Y and Z: two masses m₁ and m₂ of two different solids weremixed in an agate mortar for 15 minutes to obtain a uniform mixture. Themixture thus formed was fed to a reactor as in example D, and thecatalyst was tested as in example D.

TABLE 2 Results (comparative: pure phases - use of a single catalyst)Cata- Conversions Selectivities (%) Balances lyst m. catal. T° (in %)acetic propionic acrylic (%) Test Ex. (g) reaction C₃H₈ O₂ CO CO₂ C₃H₆acrolein acetone acid acid acid C O Ex. A1 10 0.5 355 28.3 36.0 7.2 13.69.2 0.0 1.8 9.0 0.5 58.7 100 101 388 39.9 57.8 12.9 19.1 6.3 0.0 0.5 7.30.2 53.6 100 97 Ex. A2 0.47 323 9.5 9.7 2.7 7.3 23.5 0.2 3.2 7.4 2.053.8 98 97 355 20.3 24.3 4.3 13.3 16.7 0.1 0.9 6.0 0.5 58.3 101 100 38734.5 47.2 11.3 17.1 9.3 0.0 0.3 5.3 0.2 56.5 101 100 Ex. B 5 0.5 322 4.15.1 14.3 17.7 31.5 0.1 1.6 32.1 0.3 2.6 98 99 353 8.1 11.6 19.0 23.823.5 0.1 0.7 29.6 0.1 3.0 99 99 385 14.1 22.6 25.4 29.2 17.6 0.1 1.323.0 0.1 3.2 100 98 407 18.8 32.5 29.7 33.1 14.1 0.2 0.1 19.4 0 3.5 9897 Ex. C 6 1 380 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 100 100 Ex. D 11 380 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 100 100 Ex. E 2 1 381 1.71.5 0.0 8.3 49.4 0.3 0.3 3.6 0.0 38.0 100 100 Ex. F 3 1 380 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 100 100 Ex. G 4 1 350 20.5 32.9 30.1 35 11.10.2 0.1 15.3 0.1 8.0 98 101 380 32.1 61.4 30.8 40.5 8.7 0.2 0.3 11.4 0.18.0 100 101 Ex. H 8 0.5 391 44.6 66.5 9.8 21.1 7.8 0.0 0.4 12.9 0.2 47.8101 102 Ex. I 7 0.5 386 39.6 56.6 9.2 17.2 7.7 0.0 0.3 8.4 0.2 57.1 9799 Ex. J 7 0.47 329 10.5 10.7 3.0 4.9 24.2 0.0 5.7 8.9 1.7 51.5 100 100356 19.2 22.5 5.2 7.9 17.1 0.0 1.4 9.0 0.7 58.7 99 99 392 32.9 39.8 8.916.3 10.8 0.0 0.4 7.8 0.2 55.5 99 99 406 37.8 54.9 10.6 20.3 9.4 0.3 0.07.2 0.2 52.1 99 100 Ex. K 9 1 380 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 100 100 Ex. AA1 11 0.5 390 40.6 59.2 10.8 24.7 6.9 0.0 0.1 3.8 0.153.6 100 100 Ex. AA2 0.47 323 10.0 11.2 3.5 10.5 20.5 0.1 1.7 11.0 1.251.5 100 98 354 18.6 22.9 5.0 13.6 13.9 0.1 0.7 8.8 0.5 57.3 100 99 38633.7 47.7 9.4 21.0 8.9 0.0 0.2 5.9 0.1 54.5 100 99

TABLE 3 Mechanical mixtures 0.5 g + 0.5 g unless specified ConversionsSelectivities (%) Balances Cata-yst m. catal. T° (in %) acetic propionicacrylic (%) Test Ex. (g) reaction C₃H₈ O₂ CO CO₂ C₃H₆ acrolein acetoneacid acid acid C O Ex. L 1 + 8 1 393 37.3 53.0 8.4 18.0 9.3 0.0 0.2 4.40.2 59.4 100 100 Ex. M 1 + 7 1 390 44.2 60.3 7.0 12.9 7.0 0.0 0.2 5.30.2 67.4 99 100 355 35.7 43.5 3.4 5.9 8.5 0.0 0.8 8.3 0.7 72.5 101 103323 21.1 23.6 2.3 3.0 12.4 0.0 2.6 7.4 2.3 69.9 101 103 Ex. N 6 + 7 1387 44.4 63.1 8.3 16.7 7.3 0.0 0.2 6.5 0.2 60.8 98 99 Ex. O 2 + 7 1 39045.3 62.9 7.3 14.2 6.5 0.0 0.2 5.4 0.2 66.2 99 100 Ex. P 1 + 5 1 384 6.18.2 10.9 21.4 23.7 0.1 0.3 8.2 0.2 35.2 100 100 Ex. Q 2 + 5 1 383 11.618.7 17.9 30.3 14.4 0.1 0.2 9.9 0.2 27.0 99 99 Ex. R 6 + 5 1 383 9.715.6 17.2 30.7 15.7 0.2 0.2 10.3 0.2 25.5 99 99 Ex. S 3 + 7 1 389 36.651.5 8.7 17.1 9.8 0.0 0.4 11.2 0.2 52.4 97 99 Ex. T 1 + 4 1 350 9.6 10.26.0 9.7 29.2 0.1 0.9 9.9 0.7 43.4 101 101 380 16.6 21.5 10.0 20.3 21.20.0 0.4 9.2 0.3 38.6 99 101 Ex. U 9 + 4 1 326 12.9 21.2 6.1 46.8 16.90.1 0.3 19.0 0.7 10.0 102 101 357 19.4 30.5 15.8 37.0 15.7 0.1 0.4 19.40.4 11.4 99 99 378 28.2 46.9 22.8 37.5 12.8 0.0 0.3 15.9 0.1 10.5 100 97Ex V 9 + 5 1 383 6.1 8.9 17.3 23.9 19.9 0.2 0.2 5.6 0.1 32.7 101 96 Ex W1 (67%) + 7 (33%) 1.5 353 31.3 36.9 3.3 5.7 10.8 0.0 0.7 5.0 0.8 73.7 9999 386 38.0 52.8 5.0 10.5 8.6 0.0 0.2 3.5 0.3 72.0 100 100 Ex. X 1(20%) + 7 (80%) 0.5 387 25.1 30.4 2.2 11.1 14.9 0.0 0.6 8.1 0.3 64.9 9997 (0.1 g + 0.4 g) 408 36.2 50.7 9.4 17.6 10.1 0.3 0.0 6.3 0.1 56.3 9899 Ex. Y 1 (17%) + 5 (83%) 0.6 320 4.2 5.5 11.8 19.0 25.2 0.1 1.8 28.40.8 13.0 99 99 352 7.1 10.2 16.5 23.3 22.3 0.1 0.6 17.0 0.4 20.0 100 99383 11.8 18.9 19.7 29.2 14.7 0.0 0.2 13.5 0.2 22.6 99 100 407 17.8 31.624.3 37.4 12.6 0.1 0.0 10.6 0.1 14.8 99 98 Ex. Z 1 (33%) + 5 (67%) 0.75323 4.3 5.7 11.9 20.5 25.6 0.1 1.6 25.5 0.7 14.1 98 101 352 5.8 8.4 16.923.0 20.1 0.2 0.6 14.5 0.4 24.4 98 99 385 10.1 16.3 19.0 30.4 16.3 0.20.2 9.1 0.1 24.9 99 100 407 16.5 28.9 23.4 36.2 11.8 0.1 0.2 8.2 0.120.2 100 100

1. A method for preparing acrylic acid from propane, characterized in that a gas mixture comprising propane, steam and, optionally, an inert gas and/or molecular oxygen, is passed over a catalyst conferring good selectivity, comprising a crystalline catalyst phase: satisfies the formula (I): Te_(a)Mo₁V_(b)Nb_(c)O_(x)  (I) in which a is between 0.1 and 2 limits included; b is between 0 and 1 limits included; c is between 0 and 0.2 limits included; x and y represent the quantity of oxygen bound to the other elements and depends on their oxidation states, and corresponds to a hexagonal lattice structure of which the X-ray diffraction spectrum, diffraction angles (2θ) measured using the copper Kα₁ and Kα₂ lines as an X-ray source, with a 0.02° step, has a peak at the diffraction angle 28.2° and lattice parameters a=0.729 (±0.02) nm×p, p being an integer from 1 to 4; c=0.400 (±0.01) nm×q, q being an integer from 1 to 2; α=90°, Y=120°, or of a monoclinic structure Te₂MoO₇ or Te_(0.2)MoO_(x), combined with a crystalline catalyst phase for activating the propane.
 2. A method as claimed in claim 1, characterized in that the crystalline catalyst phase for activating the propane comprises mixed metal oxides of formulae (II), (II′) or (II″): Mo_(d)V_(e)O_(y)  (II) Mo_(d′)V_(f)Sb_(g)Nb_(h)O_(w)  (II′) Mo_(d″)V_(j)Te_(j)Nb_(k)O_(z)  (II″) in which d, d′ and d″ are between 0.93 and 1 limits included; e is between 0.05 and 1 limits included; f is between 0 and 0.5 limits included; g is between 0.05 and 0.3 limits included; h is between 0.01 and 0.2 limits included; i is between 0 and 0.5 limits included; j is between 0.05 and 0.3 limits included; k is between 0.01 and 0.2 limits included; v, w and z represent the quantity of oxygen bound to the other elements and depends on their oxidation states, it being understood that the product of formula (II′) has either a hexagonal lattice structure in which the X-ray diffraction spectrum has a peak at the diffraction angle 28.2°, and lattice parameters a=0.729 (±0.02) nm×p, p being an integer from 1 to 4; c=0.400 (±0.01) nm×q, q being an integer from 1 to 2; α=90°, γ=120°, or has an orthorhombic lattice structure in which the X-ray diffraction spectrum has a peak at the diffraction angle 27.3° and lattice parameters a=2.68 (±0.04) nm; b=2.12 (±0.04) nm; c=0.401 (±0.006) nm×q′, q′ being an integer from 1 to 2; α=β=γ=90°, and it being understood that the product of formula (II″) has an orthorhombic lattice structure and also has an X-ray diffraction peak at the diffraction angle 27.3° and lattice parameters a=2.68 (±0.04) nm; b=2.12 (±0.04) nm; c=0.401 (±0.006) nm×q′, q′ being an integer from 1 to 2; α=β=γ=90°.
 3. A method as claimed in either of claims 1 and 2, characterized in that a gas mixture comprising propane, steam and, optionally, an inert gas and/or molecular oxygen, is passed over a catalyst conferring a good selectivity of formula (I) defined in claim 1, or of a monoclinic structure Te₂MoO₇ or Te_(0.2)MoO_(x) combined with a crystalline catalyst phase for activating the propane in ratios from 90/10 to 15/85 by weight of the total weight of mixture, of the catalyst conferring good selectivity/catalyst for activating the propane.
 4. A method as claimed in one of claims 1 to 3, characterized in that a gas mixture comprising propane, steam and, optionally, an inert gas and/or molecular oxygen, is passed over a catalyst conferring a good selectivity of formula (I) defined in claim 1, or of a monoclinic structure Te₂MoO₇ or Te_(0.2)MoO_(x) combined with a crystalline catalyst phase for activating the propane in ratios from 90/10 to 50/50 by weight of the total weight of mixture, of the catalyst conferring good selectivity/catalyst for activating the propane.
 5. A method as claimed in one of claims 1 to 4, characterized in that a gas mixture comprising propane, steam and, optionally, an inert gas and/or molecular oxygen, is passed over a catalyst conferring a good selectivity of formula (I) defined in claim 1, or of a monoclinic structure Te₂MoO₇ or Te_(0.2)MoO_(x) combined with a crystalline catalyst phase for activating the propane in ratios from 70/30 to 50/50 by weight of the total weight of mixture, of the catalyst conferring good selectivity/catalyst for activating the propane.
 6. A method as claimed in either of claims 1 and 2, characterized in that, when operating in the presence of molecular oxygen, the propane/molecular oxygen molar ratio in the initial gas mixture is equal to or greater than 0.3.
 7. A method as claimed in claim 6, characterized in that the propane/molecular oxygen molar ratio in the initial gas mixture is equal to or greater than 0.5.
 8. A method as claimed in either of claims 1 and 2, in which the molar proportions of the components of the initial gas mixture are: propane/O₂/inert gas/H₂O (steam)=1/0.05-3/1-10/1-10.
 9. A method as claimed in claim 8, in which the molar proportions of the components of the initial gas mixture are: propane/O₂/inert gas/H₂O (steam)=1/0.05-2/1-10/1-10. 